This invention relates to fabrication of an electrode of an electron gun and more particularly to a method of fabricating an electrode of a color picture tube electron gun, the electrode having an electron beam pass aperture and a recess surrounding the aperture.
As shown in FIG. 1, a color picture tube 1 comprises a phosphor screen 2, an electron gun 3 and a deflection coil 5. An electron beam emitted from the electron gun 3 passes through one of apertures formed in a color selective electrode 4 and impinges upon the phosphor screen 2.
Incidentally, when an electron beam emitted from the electron gun 3 is deflected toward the periphery of the screen, it undergoes distortion, so that a resulting beam spot is laterally elongated, accompanied with a halo, and focusing at the periphery of the screen tends to be degraded. Especially where the size of the screen is increased, this phenomenon will be aggravated.
To cope with the phenomenon, an expedient has been known wherein a grid electrode, for example, a second grid electrode 10 as shown in FIG. 2 is formed with a recess at a peripheral wall area of an electron beam pass aperture 18, as illustrated in FIGS. 3A and 3B. It will be appreciated herein that FIG. 2 illustrates an electron gun 3 of an in-line type color picture tube and FIGS. 3A and 3B diagrammatically detail the construction of the second grid electrode 10 used in this type of electron gun. The electron gun 3 shown in FIG. 2 comprises, in addition to the second grid electrode 10, cathodes 6, 6' and 6", a first grid electrode 9, a third grid electrode designated at 11 and 12, a fourth grid electrode 13, and bead glass members 8 for holding these electrodes.
The recess thus formed in the grid electrode to surround the electron beam pass aperture can advantageously change the shape of an electron beam such that the electron beam can be focused uniformly over the entire screen to provide a uniform beam spot. This type of electrode is disclosed in, for example, U.S. Pat. No. 4,366,414 (JP-A-59-157936).
For formation of the recess 16, a coining process utilizing a press is generally used and this technique is disclosed in, for examaple, Japanese Patent Publication No. 40-4550.
When forming the electrode 10 shown in FIGS. 3A and 3B, a substantially circular through hole 14 is typically formed in an electrode plate as shown in FIG. 4A in advance of coining with the aim of mitigating force necessary for coining, as taught from the aforementioned Japanese Patent Publication No. 40-4550. Subsequently, as shown in FIG. 4B, a recess is formed at a peripheral wall area of the through hole 14 through a coining process. Thereafter, as shown in FIG. 4C, a predetermined electron beam pass aperture 18 is formed substantially at the center of a through hole 15.
In the above prior art, however, the through hole 14 is deformed into the through hole 15 which is extremely elongated in the longitudinal direction as shown in FIG. 4B when the recess 16 is formed through the coining process. Consequently, there arises a problem that the predetermined electron beam pass aperture 18 can not be formed without accompanied with bulges 18'.
With a smaller through hole 14, a beam pass aperture 18 devoid of bulge 18' may be formed. But the smaller the through hole 14, the more the working force mitigation effect and the life of a coining tool are lessened.